RE: [Intel-gfx] [PATCH v2 01/21] drm/i915/gt: Ignore TLB invalidations on idle engines

[David Laight <David.Laight@ACULAB.COM>]

From: Mauro Carvalho Chehab
> Sent: 18 July 2022 15:54
> 
> On Mon, 18 Jul 2022 14:16:10 +0100
> Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com> wrote:
> 
> > On 14/07/2022 13:06, Mauro Carvalho Chehab wrote:
> > > From: Chris Wilson <chris.p.wilson@intel.com>
> > >
> > > Check if the device is powered down prior to any engine activity,
> > > as, on such cases, all the TLBs were already invalidated, so an
> > > explicit TLB invalidation is not needed, thus reducing the
> > > performance regression impact due to it.
> > >
> > > This becomes more significant with GuC, as it can only do so when
> > > the connection to the GuC is awake.
> > >
> > > Cc: stable@vger.kernel.org
> > > Fixes: 7938d61591d3 ("drm/i915: Flush TLBs before releasing backing store")
> >
> > Patch itself looks fine but I don't think we closed on the issue of
> > stable/fixes on this patch?
> 
> No, because TLB cache invalidation takes time and causes time outs, which
> in turn affects applications and produce Kernel warnings.

It's not only the TLB flushes that cause grief.

There is a loop that forces a write-back of all the frame buffer pages.
With a large display and some cpu (like my Ivy bridge one) that
takes long enough with pre-emption disabled that wakeup of RT processes
(and any pinned to the cpu) takes far longer than one might have
wished for.

Since some X servers request a flush every few seconds this makes
the system unusable for some workloads.

	David

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